Cellular communication systems are generally based upon the creation of specific, geographically defined, separated "cells" or "cell sites." Each cellular system is allocated a specific set of licensed frequencies (or channels) with which to operate. In a typical system, licensed voice channels are divided among seven cells, thereby providing a plurality of channels for use in each cell. This deployment is often referred to as a frequency reuse factor of seven. This seven cell cluster is then replicated, allowing for reuse of the spectrum, as needed to provide coverage of a communication area. In areas where high user volume is experienced, certain cells may be divided to form geographically smaller cells, or clusters, thereby increasing the overall number of channels available for communication.
Typically located within each cell is a base station containing control and transmission equipment. The base station generally includes at least one antenna network, associated transmit and receive apparatus and control apparatus. The base station is coupled via communication links to a mobile switch center (MSC), also often referred to as a mobile telephone switching office (MTSO), hereinafter the term MSC is used. The MSC is coupled to the local telephone switching office, referred to as the Public Switching Telephone Network (PSTN), with interfaces to standard wire line telephones. Typically, the MSC controls the operation of the cellular communication system by routing and processing the incoming and outgoing calls, and switching the calls between cell sites.
Each cell site is assigned two particular control channels, a forward control channel (FCC) and reverse control channel (RCC). The remaining channels are usually used for communications, and are called voice channels including forward voice channels (FVC) and reverse voice channels (RVC). The control channels provide commands to the mobile units for cellular control. Control transmission made from the cell site to the mobile are made on the FCC, while control transmissions made from the mobile to the cell site are made on the RCC. When a mobile telephone is activated, the telephone scans all control channels and tunes to the strongest control channel which will typically correspond to the cell site nearest the mobile user.
When a user places a call from the mobile telephone, the telephone tunes to the strongest control channel thereby identifying a particular cell and its associated base station, and the transmission is made over the RCC. The control channel and voice channels associated with a cell may be defined as "local channels." The base station at the cell site receives the transmission and sends the call to the MSC. The MSC selects a voice channel for the call and connects the call to the mobile through the PSTN. As the mobile telephone travels, the call is handed-off to a different cell site when the quality of the transmission drops below a predetermined threshold, which may occur when the mobile telephone is approaching a boundary between cells. The hand-off process is typically controlled by the MSC.
The frequency reuse approach is employed to increase the use of available spectrum while minimizing co-channel and adjacent channel interference between the cell sites. The frequency reuse factor is determined by calculating the typical propagation loss of a signal over a defined cell radius and insuring that any resulting channel interference will not result in a carrier-to-interference ratio (C/I) of less than 18 dB, under the current U.S. AMPS systems using analog frequency modulation (FM). Specific frequency assignments are determined by the characteristics of the cellular equipment (such as transceivers, filters and power combiners) and such equipment often require that frequencies within a cell be separated by as many as 21 channels. This effectively limits the capacity of the system by limiting the spectrum available for use.
The aforementioned limitations result in the allocation of a fixed frequency plan that is typically obtained from pre-derived tables. This plan results in inefficient use of the available spectrum in many instances. Channel assignments are controlled by the cellular system operator and are generally programmed into the MSC. Deployment of such a system requires that the fixed frequency plan be "fined tuned" (also referred to as "frequency reallocation"). Such fine tuning is necessary due to the inexact nature of rf propagation which in reality may vary from the predetermined calculations embodied in the fixed frequency plan. Discrepancies occur for a variety of reasons, such as weather conditions or the construction of new structures. The tuning process is time consuming and inefficient. Tuning is also required as channels are added to the cellular system or when new systems are deployed.
Prior art cellular systems have addressed the limitations of the fixed frequency plan by utilizing micro-cells. Generally, a micro-cell divides a cell site into smaller cells each serviced by a separate base station. Typically the same number of channels are assigned to each base station and thus the channels per unit area is increased. While this approach increases capacity, the number of hand-off operations is increased and interference constraints must still be considered. More importantly, this system is based upon the fixed frequency plan and again must be fine tuned.
The fixed frequency plan assumes that all assigned frequencies may be in use at a given time and that propagation characteristics are theoretical in nature. In practice however, the use of frequencies varies with time of day and day of week on a statistical basis, and many channels are available at various points in time. In addition, theoretical propagation models do not account well for structures, hills, and valleys, and many signals that are expected to propagate a set distance do not. Consequently, the spectrum is inefficiently utilized and many channels could be made available for calls, but are excluded from use in the fixed frequency plan. Prior art methods such as dynamic channel allocation and channel borrowing have been proposed and are used to help solve the problem; however, these methods require coordination and returning within the system by the system operator. Given an operators desire to increase capacity in a user dense area, there is no known method to quickly accomplish this time consuming, overall system level returning approach.
Moreover, cellular operators often need to deploy new or temporary systems to increase the coverage area or to serve high capacity needs. These capacity needs can occur during conventions, sporting events and the like, or to provide coverage in fringe areas near service boundaries and in areas of new construction. Furthermore, additional capacity and coverage is often required in areas that experience localized demand such as campus environments, airports and the like. To implement such systems, current cellular designs require not only the returning process described above, but the MSC hardware and software must first be reconfigured to accept such additions. In many cases, it is not efficient to trunk the voice traffic back to the existing MSC, nor is there time available to retune the system. In addition, proprietary interfaces between the switch and the cell site force operators to utilize equipment from the original vendor to meet these needs. There is no known method to add a small, self contained cellular system to an existing network without coordination of frequencies and interfaces to the main cellular system.
Another limitation with the current cellular system approach arises when emergency communications are required. For example, in cases of natural disasters, cellular communications are heavily relied upon by emergency response teams such as the Federal Emergency Management Agency (FEMA). In such a situation, additional capacity is quickly needed and in addition, emergency related agencies would like control of their specific frequency set. With current cellular systems this requires the deployment of small, temporary systems that are governed by the fixed frequency plan and are coordinated and controlled by the local carrier MSC. In summary, there is no known method for deploying an independent, underlay, self contained cellular system within a main cellular system that can operate on unused cellular channels without coordination or control by the local carrier or does not have the need for frequency returning.